1. Field of the Invention
The present invention relates to a television tuner capable of selectively receiving digital television signals or analog television signals.
2. Description of the Prior Art
The configuration of a television tuner according to the prior art is shown in FIG. 4. Digital television signals and analog television signals are entered into an input tuning circuit 31. The digital television signals are arrayed in other channels than those in which conventional analog television signals are arrayed, i.e. so-called vacant channels. Some sequences of television signals roughly selected by the input tuning circuit 31, after being amplified by a high frequency amplifier 32, are entered into an inter-stage tuning circuit 33. The tuning frequency of the inter-stage tuning circuit 33 and the tuning frequency of the input tuning circuit 31 are interlocked with each other. In this process, television signals on adjoining channels are further eliminated, and the selected sequence of television signals is entered into a mixer 34.
Local oscillation signals are supplied to the mixer 34 from an oscillator 35. Data for channel selection are entered into a PLL circuit 36 for controlling the oscillation frequency of the oscillator 35 and the tuning frequencies of the input tuning circuit 31 and the inter-stage tuning circuit 33. Into the mixer 34 are entered either digital television signals or analog television signals. Into the PLL circuit 36 is incorporated a switching circuit for supplying a switching signal for distinguishing between specific channels, for instance, a channel for digital television signals and a channel for analog television signals.
In the mixer 34, the entered television signals undergo frequency conversion into an intermediate frequency band (41 MHz to 47 MHz according to the U.S. specification). Therefore the converted signals (intermediate frequency signals) include those deriving from digital television signals and those deriving from analog television signals. The intermediate frequency signals are entered into an intermediate frequency amplifier 38 via an intermediate frequency tuning circuit 37. The intermediate frequency tuning circuit 37 is configured of a parallel tuning circuit, whose tuning frequency is higher than the center of the intermediate frequency band and substantially close to the video intermediate frequency (45.75 MHz according to the U.S. specification).
On the output side of the intermediate frequency amplifying circuit 38 are provided switching means 39 and two intermediate frequency circuits 40 and 41. One of the latter, the intermediate frequency circuit 40 for digital signals, processes digital television signals, while the other, the intermediate frequency circuit 41 for analog signals, processes analog television signals. The switching means 39 connects the intermediate frequency amplifier 38 to one of the intermediate frequency circuit 40 for digital signals and the intermediate frequency circuit 41 for analog signals according to a switching signal supplied from the PLL circuit 36.
Therefore, if digital television signals are entered into the mixer 34, those intermediate frequency signals are entered into the intermediate frequency circuit 40 for digital signals or, if analog television signals are entered, those intermediate frequency signals are entered into the intermediate frequency circuit 41 for analog signals.
In the configuration described above, the intermediate frequency tuning circuit is tuned to the vicinity of the video intermediate frequency. Therefore, it is convenient for the reception of analog television signals because, in video detection of analog television signals, the level of audio intermediate frequency signals should be lower than that of video intermediate frequency signals. However, in digital television signals, there are neither video intermediate frequency signals nor audio intermediate frequency signals, video signals and audio signals are superposed on each other in many carriers within the band, and accordingly it is desirable for the level deviation of the intermediate frequency band to be small. For this reason, with the aforementioned characteristic having a peak in the video intermediate frequency, the level deviation in the band is great, making it impossible for digital television signals to be correctly demodulated.
Conversely, if the intermediate frequency tuning circuit is tuned substantially to the center frequency to reduce the level deviation in the intermediate frequency band, it will be convenient for demodulating digital television signals, but the video intermediate frequency and the audio intermediate frequency being at substantially the same level would be inconvenient for demodulating analog television signals.